The invention relates generally to the field of medical imaging and in particular to a technique of extracting one or more features of interest from a radiographic image.
In the field of medical care, a wide variety of radiographic imaging modalities may be used for imaging patients. Such modalities include, for example, conventional projection X-ray systems, computed tomography (CT) systems, dual energy CT systems, tomosynthesis systems, C-arm systems, and so forth. These imaging modalities acquire projection images of an anatomy of interest within the patient. The projection images, upon reconstruction, reveal the internal structures (e.g., bones, blood vessels, soft tissues, and so forth) of the imaged anatomy due to the different densities of these structures, and resulting differences in the attenuation or absorption of incident X-rays. However, some structures are not clearly visible in the reconstructed image due to their low radio-opacity. Typically, a contrast agent may be employed during medical examinations to highlight specific structures within the imaged anatomy. Thus, vasculature and other structures may be imaged by administration of contrast agent (e.g., iodine) prior to imaging. The relative attenuation may be referred to in terms of “Hounsfield units”. Contrast agents serve temporarily to increase these values in tissues in which the contrast agents propagate.
In many applications, it is useful to extract the contrast agent enhanced structures, particularly blood vessels, for evaluation by qualified professionals. This can be done by automated or semi-automated algorithms that perform “segmentation”, or the identification of pixels or voxels in image data that correspond to locations of each tissue type. However, in the presence of contrast agents, the elevated Hounsfield units value in such tissues makes the blood vessel appear similar to bone, complicating the distinction between these. Thus, in the automatic vessel segmentation process, it becomes very difficult to differentiate bone from blood vessels, especially when the two come very close to each other in Hounsfield unit values due to the contrast agent.
Digital subtraction angiography (DSA) is one technique commonly used to automatically extract (identify and segment) contrast agent enhanced vessels. In such techniques, a mask image acquisition is performed prior to administering contrast agents to capture bone and soft tissue components in a non-contrast enhanced image. A second image acquisition is then performed after the administration of contrast agent, in which bone, soft tissue, and contrast enhanced vessels will be visible, with the vessel visibility being greatly enhanced over that of the previous image. The contrast enhanced blood vessel is then extracted by comparing the first and second images. However, there is a significant time lag between these two acquisitions, and a proper comparison requires fairly closely aligned or alignable images. DSA is therefore typically limited in application to portions of the patient's anatomy that can remain motionless for many seconds.
Alternatively, material decomposition techniques may be employed to extract the contrast agent enhanced structure directly from dual energy scan data. Dual energy systems are currently being developed that can make two or more images based upon application of different X-ray energy levels. However, material decomposition techniques do not provide robust results if the imaged object contains more than one material other than the contrast as the technique targets material having fixed chemical composition. For example, bones and contrast agent enhanced vessels may be isolated directly using material decomposition. However, the bones may be of different chemical compositions in a human body. Thus, material decomposition that works on a given target chemical composition will fail to account for all types of bones. The use of present techniques has simply not provided the ability to distinguish between all three types of tissues, bone, soft tissue, and contrast-enhanced vessel tissue.
It is therefore desirable to provide an efficient technique for extracting contrast-enhanced structures (e.g., blood vessels) from a radiographic image with improved accuracy and reliability.